1. Field of Invention
The present invention concerns a process for manufacturing a high strength rail and apparatus to carry out this process. This process comprises a thermal treatment of the rail as soon as it comes out of the rolling mill, i.e. at the rolling heat.
2. Description of Prior Art
Due to the present trend to increase the loads and the speed of trains, the rails are subjected to ever more severe stresses, which require ever superior properties. In this respect, it is particularly important that the rails are as perfectly straight as possible and have a high level of resistance to wear, resistance to fracturing, ductility, resistance to fatigue and shock, and hardness. Finally, they must have a satisfactory weldability.
From the economic point of view, it is still advisable to keep their price reasonable, in particular by avoiding or limiting the use of alloying elements.
The mechanical properties mentioned above are particularly important in the rail flange, since it is this part, and particularly its upper region, which is subjected to the highest stresses, in particular of wear and shock.
It is known that, in order to have the requisite properties, the rail flange must be made of fine perlite free from pro-eutectoid ferrite and martensite, and possibly containing a low percentage of bainite, and that furthermore, the gradient of hardness in the flange is preferably as gentle as possible.
The steels used for the manufacture of high strength rails generally contain 0.4% to 0.85% of carbon, 0.4% to 1% manganese, and 0.1 to 0.4% silicon, the rest consisting principally of iron.
In a prior proposal which is the subject of the Belgian Pat. No. 889 617 in particular, the present applicant described a process consisting in adjusting the length of the cooling ramp, the speed of travel of the rail and the average density of the heat fluxes applied to the head, the web, and the flange of the rail in such a way that martensite does not form in any part of the section of the head, and that less than 60% of the section of the head has undergone austenite-perlite allotropic transformation on leaving the cooling ramp.
With this process it is possible to make very straight rails economically, having the required properties, in particular a Brinell hardness number of the order of 380, with steels having the composition cited above.
In fact, however, this degree of hardness is no loner adequate in every case, and users demand higher and higher Brinell hardness numbers of around 400.
It is not possible to meet this new demand with the above-mentioned process, which cannot produce sufficient cooling to achieve the required hardness without the formation of martensite.
Efforts have therefore been made to increase the hardness of the flange by adding alloying elements to the steel, for example 0.1% to 0.5% chromium and up to 1% silicon.
However, it has been shown that it is not possible to obtain the desired result by means of such an addition, i.e. a Brinell hardness number of 400 without the formation of martensite in the flange, when applying the process cited above. To obtain this result, it would in fact be necessary to reduce the cooling intensity considerably to a level incompatible with the process and apparatus of the Belgian patent cited above, and at the same time to increase the length of the cooling process by a considerable degree. The latter should, for example, be five times longer, which would lead to either a corresponding reduction in the speed of the rail, or an increase in the length of the cooling ramp and hence a blockage of the plant, involving futher necessary outlay.
Furthermore, it is not desirable to reduce the speed of the rail by a great degree, as this would result in the tail of the rail remaining too long in the air and the beginning of allotropic transformation before the start of controlled cooling.